optional arguments

[Jonathan A Rees <JAR@AI.AI.MIT.EDU>]

I have a different optional arguments proposal, which people should keep
in mind as an alternative:

Don't make any change to the syntax of LAMBDA.  Instead, just introduce
a new special form for taking apart rest arguments.  Example:

  (LAMBDA (A B . R)
    (OPTIONAL ((X 1) (Y (+ X 5))) R
      -body-))

would be analogous to the Comon Lisp lambda-expression

  (LAMBDA (A B &OPTIONAL (X 1) (Y (+ X 5)))
    -body-),

and

  (LAMBDA (A B . R)
    (OPTIONAL ((X 1) (Y (+ X 5)) . R) R
      -body-))

would be like

  (LAMBDA (A B &OPTIONAL (X 1) (Y (+ X 5)) &REST R)
    -body-),

I think this gets most of the benefits you want without making LAMBDA
hairy.  It provides parameter list destructuring and error checking in a
nice orthogonal way, and is only a little bit more verbose than hairy
LAMBDA-isms.  And it can be implemented in straightforwardly as a macro.

I have had this in the back of my mind since about 1981.  I never got
around to installing it in T, but I should have.  I have implemented and
used it on several occasions (e.g. for implementing R^2 Scheme in T),
and was fairly happy with it.

Effiency note: the mythical "sufficiently clever compiler" can avoid
consing the rest-list if there's only the one reference to R.  I think
this would be a very straightforward transformation.

Jonathan